public class PalindromeList {
    public class ListNode {
        int val;
        ListNode next = null;

        ListNode(int val) {
            this.val = val;
        }
    }

    public boolean chkPalindrome(ListNode head) {
        if (head == null) {
            return true;
        }

        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        ListNode cur = slow.next;
        while (cur != null) {
            ListNode curNext = cur.next;
            cur.next = slow;
            slow = cur;
            cur = curNext;
        }

        while (slow != head) {
            if (slow.val != head.val) {
                return false;
            }
            if (head.next == slow) {
                return true;
            }
            slow = slow.next;
            head = head.next;
        }
        return true;
    }
}
